Ceramic mixed materials are mostly produced from corresponding oxides and carbonates, which for this purpose are ground intensively in ball mills, mixed, and then calcined at high temperatures. These processes do, however, require that a heterogeneous mixture be prepared, which undergoes reaction only incompletely to form a homogeneous phase, as a result of the calcining step.
The present-day, high-performance ceramics require extremely uniform distribution of elements without local accumulation of the principal components and dopants.
Increasing numbers of applications are using piezoelectric or corresponding ferroelectric properties of ceramic materials. For instance, distance measuring devices, ultrasound transducers, inkjet nozzles, and common-rail diesel injectors are being produced from high-performance ceramics. The ceramics presently used here are produced on the basis of lead zirconium titanate (PZT) and dopants.
In view of the increasing importance of the environmental compatibility of such actuators and resonators, efforts exist to eliminate components detrimental to health and to the environment from these electrical systems.
An alternative system of materials with piezoelectric properties is based on sodium potassium niobate. The piezoelectric properties of ceramic components made from this system of materials are dependent in particular (in addition to powder properties typically important for ceramics production, such as particle size or, correspondingly, particle size distribution, chemical purity, sintering activity, contraction, and so on) on the density, phase purity and/or chemical homogeneity, and complete or near-complete orientation of all of the ferroelectric and/or piezoelectric domains of the ceramic particles into an external preferential direction. This complete orientability of the domains is achieved in particular through the defined orientation of the maximum number of grains to an external electrical field. This electrical field produces a polarization.
Such orientation of the grains to the electrical field is achieved through the orientation of suitable platelet-shaped and/or needle-shaped single crystals during film/tape casting and during subsequent heteroepitactic or homoepitactic growth of the desired (pseudo)cubic phase from the sodium potassium niobate system of materials.
Suitable platelet- or corresponding needle-shaped crystallites form the precondition for the orientability of the grains in the ceramic or of the corresponding ferroelectric domains in these grains. These crystallites are obtainable by elaborate culturing of single crystals or by directed production during the synthesis of the ceramic powder.
In the state of the art, these ceramics are used for producing piezoelectric components. These components are an important element, for example, in mechatronics. Components of this kind are used particularly in diesel injection assemblies for powerful diesel engines with low emissions. The requirements imposed on such multilayer structures made of piezoelectric ceramics are enormously high.
The switching strokes are becoming ever larger and must be made with more rapid switching, and the function must also be able to be ensured under the extreme ambient conditions within the engine.
The poor recyclability of these components and the toxic lead material used has led to the search for piezoelectric materials which have technical properties similar to or better than those of PZT. One technical property is the piezoelectric coefficient d33, which is indicated with the units pm/V. This piezoelectric coefficient is a measure of the longitudinal extent per volt of voltage over a metallized and poled ceramic sample in the small-signal range in the direction of the applied field.
In 2004, the automaker Toyota together with the supplier Denso published a composition for a piezoelectric ceramic in the journal Nature under the title “Lead free piezoceramics”, Vol. 432 (2004), pp. 84-87, this ceramic having properties comparable with those of the present-day high-performance PZTs (Pb(ZrxTi1-x)O3).
This material consists of complex perovskites AA′BB′O3 with essentially potassium and sodium at the A-site and niobium and tantalum at the B-site. An added dopant at the A-site is Li, and at the B-site Sb. The phase, moreover, is configured in platelet shape by a complicated template method. This means that there is a specific morphological orientation, by means of which the piezoelectric coefficient d33 has been raised from around 250 pm/V to the PZT-comparable figure of 400 pm/V. The compound was prepared from different oxides (Nb2O5, Ta2O5) and carbonates (K2CO3, Na2CO3). This method is described in references including patents EP 138 2 588 and DE 102 005 027 928 and in the “Lead free piezoceramics” writeup in Nature 432 (2004) 84-87.
The inadequate miscibility of the patented niobium and tantalum compounds in the mixed-oxide method of Toyota and Denso is described in the article by Yiping Guo in Mater. Lett. 59 (2005), 241-244.
Older preparation prossesses exist for pure potassium niobate or sodium niobate with the particles having no special particle form. In the Russian journal I2v. Vysshikh Uchebn. Zavedenii Tsvetn. Met. (Nonferrous Metallurgy), 5 (1963), pp. 99-107, Zelikman describes the reaction of potassium hydroxide with niobium hydroxide in an autoclave at 150° C.-200° C. under pressure to give a soluble potassium compound (K8Nb6O19), which by calcining at above 400° C. or with high KOH concentrations decomposes to form pure potassium niobate. This citation is also referenced by Gmelins Handbuch der anorganischen Chemie, Niobium, Part B4, Edition 8, 1974, p. 157.
The use of an autoclave and of the high temperatures for this process gives rise to high costs.
In the recovery of niobium and tantalum from ores and ore concentrates, there are processes known in which niobium oxides and tantalum oxides are leached out using concentrated alkalis (e.g., Hydrometallurgy 80, (2005), pp. 126-131; CN 1238536; JP 8041559).
Similar processes are used in the hydrothermal synthesis of potassium and sodium niobates, in which niobium oxide is digested in an autoclave with alkali metal hydroxides; C. H. Lu, Mater. Lett. 34 (1998) 172-176.
A further process is based on the hydrolysis of niobium and tantalum oxides. DE 125 7 125 to CIBA is one of the references describing this process. More in-depth articles include “microemulsion mediated synthesis of nanocrystalline (KxNa1-x)NBO3 powders”, J. Crystal Growth, 280 (2005) 191-200, which uses oil-in-water emulsions to control the particle size.
In order to obtain soluble compounds, it is also possible to employ complexes of the niobium and tantalum oxides. In this case, tartrates and peroxotartrates play a predominant role among the carboxylic acids. These complexes are subsequently decomposed at high temperatures to form the target compounds (niobates). Examples of this can be found in M. Devillers, Inorg. Chem. 44, (2005), pp. 1554-1562 or B. Malic, J. Eur. Ceram. Soc. 25, (2005), pp. 2707-2711.
In the case as well of the use of sodium potassium niobate (also called NKN below) as a system of materials for piezoelectric components, the piezoelectric parameters are dependent on particle size, purity, sintering activity, contraction, and the like, and also on homogeneity. A greater homogeneity is achieved by higher calcining temperatures. In the case of NKN, however, this leads to evaporation of the volatile potassium oxide.